1. Field of the Invention
This invention relates in general to thin film magnetoresistive reproduce heads of the type having oriented easy magnetization axes; and in particular to such heads, and apparatus cooperative therewith, and methods in which signal distortion and head noise are minimized and/or eliminated.
2. Description Relative to the Prior Art
Playback of recorded signals from a magnetic recording medium by means of a conventional magnetic head that is sensitive to flux rate-of-change is difficult for low recorded signal frequencies, and theoretically impossible for recorded DC signals, or when there is no relative speed between the recording medium and the magnetic head. Accordingly, various techniques have been proposed for sensing magnetic flux, as opposed to the rate-of-change of such flux, e.g. Hall effect, flux gate, and magnetoresistive devices, thereby to permit recording media to be used as optimally as possible.
With regard to the matter of magnetoresistive devices for sensing magnetic fields, the use of a thin film magnetoresistive structure for field sensing purposes is known. Representative prior art depicting the use of such magnetoresistive thin film structures can be found in the following references: U.S. Pat. Nos. 3,731,007; 3,947,889; 3,921,218; 3,945,038; and 4,051,542. Typically, a thin (planar) magnetoresistive film is employed to sense a magnetic field to which it is exposed by passing an electric sense current (either AC or DC) through the film, the film magnetization vector being canted with respect to both the direction of current flow and the direction of the field being sensed. The field being sensed exerts a torque on the magnetic moment in the film, causing the resistance of the film to increase or decrease depending on the sense and magnitude of the field applied to the film. The resistance of the film is, therefore, the analog of the field strength. Although such use of a magnetoresistive film represents a reasonably workable technique in the art of magnetic flux sensing, it suffers from, among other things, noise susceptibility and distortion:
As is known (Barkhausen) noise associated with a magnetic structure increases in direct proportion to the strength of any magnetic field applied to it, the magnetic domain walls within the structure abruptly migrating about in a noisy fashion as a function of the strength of such applied field. With regard to the matter of distortion, since the magnetization vector of a thin film magnetoresistive structure is canted (or cantable) with respect to both the direction of sense current flow and the direction of a field being sensed, variation in the magnetoresistance of the structure is inherently non-linear, i.e. it varies as a function of the cosine of the angle between the current direction and the magnetization vector, as well as a function of the cosine of the angle between the magnetization vector and the direction of the applied field being sensed. Such non-linearity undesirably gives rise to the generation of even harmonics of any alternating signal analog produced by the thin film structure. Also, when in response to large swings of an alternating magnetic field applied to the thin film magnetoresistive structure the structure periodically saturates (i.e. the cant of its magnetization axis remains fixed in orientation despite further increases in the strength of the field applied to it), odd harmonics of the corresponding signal analog are also undesirably produced. Harmonics, whether odd or even, appear as distortions of and within the signal analogs.
Although compensation for the non-linearity of the magnetoresistance-versus-field response characteristics of a thin film structure by precluding the production of even harmonics may be achieved relatively easily by use of paired oppositely-acting thin film structures in a bridge circuit as taught in U.S. application Ser. No. 057,619, now U.S. Pat. No. 4,306,215, such a technique inherently increases the production of noise and the generation of odd harmonics.
A paper has recently been presented (IEEE Transactions on Magnetics, Vol. MAG-15, No. 6, November 1979, "A Magnetoresistive Head With Magnetic Feedback," pages 1625-1627, by E. de Niet and R. Vreeken) disclosing a technique for precluding the generation of harmonics, and vastly reducing noise, in a thin magnetoresistive head. Such technique, as will be appreciated below, is (at least broadly) conceptually the same as that practiced by the invention.